Short GRB 160821B: A reverse shock, a refreshed shock, and a well-sampled kilonova

G. P. Lamb, N. R. Tanvir, A. J. Levan, A. De Ugarte Postigo, K. Kawaguchi, A. Corsi, P. A. Evans, B. Gompertz, D. B. Malesani, K. L. Page, K. Wiersema, S. Rosswog, M. Shibata, M. Tanaka, A. J. Vander Horst, Z. Cano, J. P.U. Fynbo, A. S. Fruchter, J. Greiner, K. E. HeintzA. Higgins, J. Hjorth, L. Izzo, P. Jakobsson, D. A. Kann, P. T. O'Brien, D. A. Perley, E. Pian, G. Pugliese, R. L.C. Starling, C. C. Thone, D. Watson, R. A.M.J. Wijers, D. Xu

Research output: Contribution to journalArticlepeer-review


We report our identification of the optical afterglow and host galaxy of the short-duration gammaray burst sGRB 160821B. The spectroscopic redshift of the host is z = 0:162, making it one of the lowest redshift sGRBs identified by Swift. Our intensive follow-up campaign using a range of groundbased facilities as well as HST, XMM-Newton and Swift, shows evidence for a late-time excess of optical and near-infrared emission in addition to a complex afterglow. The afterglow light-curve at X-ray frequencies reveals a narrow jet, ∼ 1.9+0.10-0.03 deg, that is refreshed at >1 day post-burst by a slower outow with significantly more energy than the initial outow that produced the main GRB. Observations of the 5 GHz radio afterglow shows a reverse shock into a mildly magnetised shell. The optical and near-infrared excess is fainter than AT2017gfo associated with GW170817, and is well explained by a kilonova with dynamic ejecta mass Mdyn = (1.0-0.6) × 10-3Mo and a secular (postmerger) ejecta mass with Mpm = (1.0-0.6) × 10-2Mo, consistent with a binary neutron star merger resulting in a short-lived massive neutron star. This optical and near-infrared dataset provides the best-sampled kilonova light-curve without a gravitational wave trigger to date.

Original languageEnglish
JournalUnknown Journal
StatePublished - May 6 2019


  • (Stars:) gamma-ray burst: Individual GRB 160821B
  • stars: Neutron

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